The invention relates to a method for fabricating a semiconductor device and, more particularly, to a method for fabricating a semiconductor device in which a lifting phenomenon can be prevented from occurring in forming an amorphous carbon film on an etched layer having tensile stress.
As semiconductor devices have become more highly integrated, the devices have to be formed in a high density on a predetermined cell area and therefore unit devices such as a transistors and capacitors, etc. decrease in size. In particular, as a design rule decreases in a memory device such as a flash memory, semiconductor devices that are formed inside cells decrease generally in size. Recently, a minimum line width of a flash memory device is formed equal to or less than 0.1 μm, and further as required to be equal to or less than 60 nm. Therefore, several problems arise in fabricating semiconductor devices forming a cell and various solutions has been tried to solve the aforementioned problems.
In particular, as to a step of forming a pattern of metal wire, etc., in the prior art, a reflection preventing film was formed of SiON, etc., and a hard mask was formed of a nitride film such as SiN, etc., and then a photolithography process was performed using such resulting films. However, there is a limitation to the step of fabricating a semiconductor device having a minute line width equal to or less than 60 nm. As a result, a solution has been proposed, in which a reflection preventing film and a hard mask are formed at the same time using a amorphous carbon film and photolithography is performed in order to form a minute pattern in fabricating a semiconductor device having a line width equal to or less than 60 nm.
However, when the reflection preventing film and the hard mask are formed using a amorphous carbon film, a problem arises in that a lifting phenomenon occurs, which does not occur in forming the reflection preventing film and the hard mask using SiON and SiN.
Here, this lifting phenomenon relates to a stress concentration factor Kc on a stacked layer. Typically, when the stress concentration factor on a stacked layer is large, the lifting phenomenon may occur due to a low interfacial bonding force. However, when the stress concentration factor is small on the stacked layer, the lifting phenomenon may not occur due to a high interfacial bonding force. In addition, the stress concentration factor is proportional to a square root of the stress applied to a film multiplied by a thickness thereof and the total stress concentration factor of a stacked layer is a sum of the respective stress concentration factors on each layer. The stress concentration factor is represented by a formula as follows.Kc=Ω×stress×thickness 2 (Ω=1.46)  [Formula 1]
The following Table 1 shows stresses and stress concentration factors on an amorphous carbon film and a tungsten layer formed by a Chemical Vapor Deposition (hereinafter referred to as “CVD”), respectively, and a stress concentration factor on stacked layers formed by stacking them, according to the prior art. In addition, FIG. 1 is a sectional view showing the aforementioned-stacked layer.
TABLE 1FilmThickness (Å)Stress (dyn/cm2)Kc (MPa/m0.5)Amorphous carbon15009.00e80.051filmCVD tungsten film8001.5e100.619Total Kc on the Stacked layer0.67
Generally, the CVD tungsten film that is used as a metal wire of a flash memory has a large tensile stress of 2000 MPa (2e10 dyn/cm2) and the amorphous carbon film has a large tensile stress of 90 MPa (0.9e9 dyn/cm2) at a forming temperature of 550° C. Therefore, when an amorphous carbon film is formed over a CVD tungsten film to form a metal wire of a flash memory, two layers having large tensile stresses, respectively, are stacked. As a result, as shown in Table 1, the stress concentration factor becomes relatively large and thus a lifting phenomenon occurs on an interface, as shown at portion A of FIG. 1.
To solve the problem of a lifting phenomenon occurring on an interface, a tungsten film formed by Physical Vapor Deposition (hereinafter referred to as “PVD”), rather than CVD tungsten film having a large tensile stress, has been used.
The following Table 2 shows stresses and stress concentration factors on an amorphous carbon film, a PVD tungsten layer, and PE nitride film (Plasma Enhanced nitride), respectively, and a stress concentration factor on stacked layers formed by stacking them. In addition, FIG. 2 is a sectional view showing the aforementioned-stacked layer.
TABLE 2FilmThickness (Å)Stress (dyn/cm2)Kc (MPa/m0.5)Amorphous carbon20009.00e80.059filmPE nitride film300−2.60e9−0.066PVD tungsten film5.00e90.206Total Kc on the Stacked layer0.199
Referring to Table 2, since the value of a stress concentration factor on the stacked layer formed, as the aforementioned way is relatively small, a lifting phenomenon does not occur, as shown in FIG. 2. However, since a burial step of a metal contact is difficult when a PVD tungsten film is used, in order to perform a step of using a PVD tungsten film, a prior CVD tungsten film is buried to form a metal contact and then a metal wire is formed using PVD tungsten. Accordingly, the numbers of steps increase to become complicate in using PVD tungsten.